


The Doppelganger

by Myfanwi



Category: Undertale (Video Game)
Genre: Alternate Universe - Horrortale (Undertale), Alternate Universe - Underfell (Undertale), Angst and Humor, Crossover, Gen, Horrortale Papyrus (Undertale), Horrortale Sans (Undertale), Underfell Alphys (Undertale), Underfell Frisk (Undertale), Underfell Papyrus (Undertale), Underfell Sans (Undertale), Underfell Undyne (Undertale)
Language: English
Status: Completed
Published: 2021-02-14
Updated: 2021-02-25
Packaged: 2021-03-14 22:20:46
Rating: Teen And Up Audiences
Warnings: Graphic Depictions Of Violence
Chapters: 13
Words: 9,360
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29425908
Author URL: https://archiveofourown.org/users/Myfanwi/pseuds/Myfanwi
Summary: [Short story - Underfell & Horrortale crossover] To wake up in the skin of someone else : nightmare or reality ? That is the question asking by two Sans, so close but so different from each other. Both send in another universe they don't know anything about, they will have to struggle to get back home by theirselves. But careful, danger is never far away.
Comments: 4
Kudos: 21





	1. Chapter 1

**Author's Note:**

> Hello ! I have this old fanfiction in 13 parts that I wanted to share to you. That’s a Underfell and Horrortale crossover where Underfell Sans and Horrortale Sans switch places for some reason. The updates will come quickly, I just have to translate the story, since it’s originally a french fanfiction ! It’s mostly a humoristic story, but there are some soft gorey parts. Well, it’s Horrortale, you know, it’s not really a happy universe.
> 
> Disclaimer : The world and the characters belong to their original creators. I don’t earn money on my writing.

# THE DOPPELGANGER

### Part 1 : Underfell

— Ya’ sure ya find it like that ? Damn ! That’s not a small hole. Ya’ know, I’m not sure even Alphys can fix this. That’s sure ugly.  
— Undyne, could you stop rhapsodize and help me carry him ? He’s heavy !

Sans half-opened his eyes. His skull hurt him like someone crushes a bottle on it. He felt nauseous but it didn’t take long for him to understand why. Arms and legs up, two monsters was lifting him with more or less difficulties. He struggled a little to free one arm and reached the hole in his head. Old habit. His hand crossed briefly the hole and he sighed. The scar was still here. Still no reset then. So what was happening ?

He rotated the head to look around him. The Royal Guard caught it ? They were in Waterfall ! That couldn’t be good. And Papyrus ? Where was Papyrus ? Distraught, he lifted his head to see better who exactly was holding him. That was troubling. Even if he had sharp teeth and ridiculous shoulder pads, he swore that guy looked like his brother. Weird. He was insane, but not at the point he didn’t remembered what his brother looked like the last time he saw him. And even if he was him, where the hell could he even find an armor like that ? All armors of the Underground were stored in the castle.

— Pap’ ? he moaned.

« Papyrus » looked down in his direction, surprised to see him already awaken. His face got instantly cold and detached, almost evil.

— Shut up, Sans. I don’t want to hear your stupid excuses right now. Sleep. You are… badly damaged. Even Asgore would be scared of you with a head like this.  
— A… Asgore ? he replied, confused.

But Asgore died years ago. Why this sudden melancholy ? Sans got more and more the impression he didn’t belong in this place. That feeling badly reinforce when his eyes turned towards the neck of the second person holding him. What the hell ?! He violently struggled, surprising his two carriers who dropped him on the floor. Sans evoked a bone wall and retreated to put the maximum distance between his brother, him and her. Panicked, he probed the ground to reach his axe. The bone wall consumed all his day magic in once. He wouldn’t be able to defend Papyrus like that. 

— The hell is wrong with h…

Undyne couldn’t finish her sentence. To protect Papyrus, Sans jumped on her and went directly for her face. He hit her until his phalanges started to hurt. The fish lady tried to repulse him, completely lost, while his brother kept his arms dangling, too shocked by what was happening in front of his eyes to act and do something. The small skeleton planted all his teeth in his prey’s shoulder, blinded by madness. Undyne screamed in pain as Papyrus finally reacted.

He grabbed Sans under the arms and dragged him back. The skeleton kept fighting fiercely and growling like an animal, his only eye injected with blood. A big red bone maintained it under the throat so Papyrus could hold him as best he could. Sans yelled like a lunatic, still obsessed with the idea to kill Undyne, whatever the price. 

— Fuck this shit ! she yelled.

She invoked one of her spear and smashed it against the small skeleton head, as much for revenge as for knock him out. The effect was immediate and he fell inert in his brother’s arms. Papyrus was out of breath. 

The two royal guardsmen exchanged a worry blink. What in the world just happened to Sans ?


	2. Part 2 : Horrortale

**Summary for the Chapter:**

> In another universe, an other Sans is also waking up in the snow. Is Snowdin colder or something ?

# The Doppelganger

###  _Part 2 : Horrortale_

Sans woke up with the worst headache of his life. Everything was dizzy and nauseous around him. He got away of the pile of snow where he was burry and got back on his trembling legs, a bit lost. Was it him or Snowdin was much colder than usual ? Even with his heavy black jacket he caught himself shivering. He looked around him, searching for something he could recognize. He didn’t know this part of the forest existed before. How the hell did he even get here in the first place ? Did he sleepwalking again ? Or Boss threw him out for some reason ? He had no idea or memory of what could have happen to him.

— Shit. I must have push too hard on mustard, he growled as the world spinned around him. Boss gonna kill me.

He rested on a dead tree for a second and tried to understand where was his home in this vegetal labyrinth. He didn’t know what time it was, but if he missed his work duty, there was no way he would reach Snowdin alive anyways. Boss was probably patrolling and searching for him. If he didn’t find him first, Undyne would have his head anyways. He forgot he had a report to write for today. He already had two warnings this week. A third one and he was good to a special one-o-one meeting with Asgore. No one would that, and not him above all that.

But it was still weird. He must have lie down here for a few hours and no one tried to kill him ? Welp, it should be Boss then. When this idiot raised his voice, all of Snowdin’s morons ran away in fear. Or maybe everyone thought he was already dead anyways. Bunch of morons.

He sighed and progressed a bit in the middle of the trees. He had to find the main road. His baskets sank in the cold white powder. It almost seemed like this snow was here for months. Was it already winter on the Surface ? But yesterday again, he swore it was hot like in the middle of the summer. The CORE probably had some problems again with the climate regulation. If only this little piece of precious jewelry was not resting in the hands of a sociopath doubled by a failed mad scientist, maybe it won’t happened. But no, no ! Nobody ever listen this good old Sans. All eyes were on the great and terrible Papyrus and he had to be content with crumbs. Ungrateful jackass. And to think he changed the diapers of this brat !

Something passed brusquely at a run, somewhere in front of him before sinking in the depth of the forest. He froze, suddenly worry. Was he heading in an embush ? That was his luck. In his state, he wouldn’t stand long against an angry gang of monsters. How could he be always the one in this sort of situations ? And where the hell was Papyrus when he needed him ? He growled and invoked some bones around him, uneasy.

— Well, listen, ya’ puppy scraping… Instead of turning around like vultures, show yourselves and attack me already ! Ya’ cowards !

Only silence answered him. Yeah, as usual. But he didn’t see the huge shadow who leaned above him and its long, long sharp teeth similar to razor blades.

— Good morning, Sans.

Sans shrieked and stepped back in alarm. He fell on his butt in the snow and lifted his head to stare at his brother. Or was he really his brother ? Papyrus didn’t have the same face than the day before. He was wearing a white dirty costume that inflated his chest like a balloon. He still had his red scarf, even somehow it looked like the scarf was sad. It hung pitifully the long of his neck, like some sort of dead snake. Papyrus stood bend behind him in a strange manner. His eyes seemed smaller, but the mess the teeth were doing on his jaw caught his attention more. His teeth were sharp, oriented in multiple directions and covered with garbage. Scary.

That thing couldn’t be his brother. That was nothing like him.

— Sans ! You got your eye back ! But I thought Undyne stole it ?  
— Undyne ? What ? he asked, lost. Um… B-Boss ? What is going on ? Is this already Halloween ? But we’re in the middle of winter ?  
— Hallo-what ? Sans, are you sure you’re ok ? You are looking pale. Well, I mean, more than usual, since we’re skeletons. But no time for that ! he yelled, changing the subject. Sans ! Sans ! I caught a human ! All by myself ! I dropped it at Grillby’s and run to get you. Are you coming ? You didn’t eat nothing in two weeks !

The huge skeleton pulled his arm in excitement, then dragged him behind, forcing him to run to avoid falling. Sans had no idea what was going on. He had to be stuck in some sort of nightmare. But a very realistic one in that case. It almost looked like a parallel world. Even Papyrus was nice to him. Was he dead ? Maybe Boss hit him too hard on the skull or something and he got ejected from existence. Well, that was the only credible thing he found at least.


	3. Part 3 : Underfell

**Summary for the Chapter:**

> Meanwhile, in Underfell, Papyrus, Undyne and Alphys are still invistigating to understand what happened to Sans. Well... Good luck with that.

#  **THE DOPPELGANGER**

###  _Part 3 : Underfell_

Despite the iron ties that kept the tiny skeleton on the lab table, Undyne was still fearing for her life. He was yelling and kicking, enraged, while yelling insults at her. She might find herself behind a armored window, she didn’t feel more safe than when she carry him here in the first place. Sans was giving her the chips. For real. A look towards Papyrus learned her that it was the same thing for him.

Alphys was trying the best she could to administre the skeleton a sedative, but the needle always hit the bones next to his soul as Sans struggled on the table. Even Mettafell came to the rescue. His four mecanic arms was placarding the skeleton arms and legs against the metal, with a lot of difficulty. 

— Ya’ sure he doesn’t have rabies, right ? asked Undyne for the umpteenth time while touching her shoulder’s bandage. I don’t want to become like him. He’s creeping me out. Shit, Pap’, what the hell happened to him ?  
— I don’t know. Last night he was sleeping in his bedroom, but this morning, he wasn’t there anymore. I found him like that in the middle of the forest, with that… That hole on the top of his skull.   
— Well… If someone told me yesterday you just had to hit his skull hard enough so he finally understand how things are working down there, I wouldn’t have believe it. Maybe ya’ should have done it sooner.  
— Undyne, I don’t think he is my brother.

Undyne puffed.

— And who else would that be ? Do you know any other skeleton ? That’s Sans. He’s just… very pissed against me for some reason. But seriously, I didn’t see him in three weeks, I promise you I don’t understand why he wants to kill me. Maybe some kids attacked him and he gone totally insane ? Not to be mean or anything, but the guy already had problems before so…  
— I just told you that he is not my brother. Are you even listening ? I live everyday with that worthless lazy shit, and that thing is not Sans.  
— Be careful. A little more and I swore you were worry for a second.  
— Shut up, Undyne.

Mettafell suddenly yelled. Sans managed to catch his arm and was biting in it at full force. Alphys took her chance and finally plant the needle in his soul. The skeleton calmed down instantly and the royal guards were finally able to enter the room. The scientist quickly inspected Sans’ head while he was still knocked out. The hole was very ugly, like someone cut directly in it with a sword. Soon enough, Alphys frowned.

— You said he did that this night ?  
— Affimative, answered Papyrus. He was perfectly fine yesterday.  
— That’s not possible. Given the healing of some pieces of his skull, that wound was done several years ago. Five or six years ago, I think. You are sure Sans doesn’t have a twin or something ? He is looking thicker and bigger than usual. And… You know ? More evil. Not that Sans is not evil, but, well… He barely knows how to defend himself. Or maybe, that’s a doppelganger.  
— A what ? asked Undyne.  
— That’s… That’s like in those animes. Doppelgangers are taking the form of someone his victim knows. Then, they kill their clone and take their place and act like nothing ever happened. Most of the time, they ended killing everyone their clone loved then start again with a new prey. Maybe that’s not Sans. Maybe he even killed the other Sans. That’s not that hard, he only has one HP. Who knows ?

Undyne’s eyes shined with excitement.

— That’s so cool ! I want a dopp… dapp… a doppelginger too ! And fight with it with the biggest sword I can find !  
— Oh, purred Alphys, suddenly interested. We can do that, I have plenty of ideas of what we could do with that sword. Are you free tonight ?

Papyrus released an exasperated sigh. He took a step forward and place himself between the two women. He crossed his arms and showed Sans with his head.

— So that’s not my brother, we are all agree on this. But then, what is that ? And where is my brother ?  
— Ya’ hear Alphys, right ? He probably got eaten or something even more gore. No one cares ! This Sans is way cooler anyway. I’m taking him in the Royal Guard, I’m sure he will make people shit themselves during interrogations ! Look at his head ! He’s even scarier than you !  
— … Thanks, Undyne, he said, slightly upset. But unfortunately, no, my brother is still alive. If he died, I would have felt it. I would like to have him back before this... roppelginger take care of him. Or whatever this thing is.   
— Ya’re not funny, Papyrus, growled Undyne. What ya’ even want to do anyway ? Draw a path made of mustard bottles and wait for him to find his way to your home ?

Papyrus didn’t answered. His face closed as the idea went from one part of his skull to the other.

— But what if it’s working ? he said with a large grin.


	4. Part 4 : Horrortale

**Summary for the Chapter:**

> In Horrortale, Sans thinks he's doing a bad trip. Just wait to see what is waiting him in Snowdin Town.

#  **THE DOPPELGANGER**

###  _Part 4 : Horrortale_

Dragging by a very excited Papyrus, Sans entered Snowdin with a bit of apprehension. He immediately noticed something was off. The town was strangely quiet. Half of the houses were falling apart and almost all the windows had been smashed. Some monsters wandered here and there, starving, looking at the snow with their blankly stare. When Sans appeared, it was almost like he was scaring them somehow : all of them fled his gaze with a fearful expression. Disturbed, he sped up to stay close to his « brother ». 

Papyrus suddenly stopped and Sans bump into his legs. They were in front of Grillby’s. As usual, the place seemed crowded. Maybe a little more than usual. The tall skeleton turned towards him with a bit of contrariety.

— Hurry up, Sans ! There won’t be anything for us !  
— Ya’, ya’. Hey Boss, what’s their problem ? he asked after a rabbit lady shrieked and hide when she saw him. Did ya’ yelled after them or something ? ‘Seems like they’re giving us the… cold shoulder.

Papyrus bent down in his direction. Sans felt very uneasy in front of these big pointy teeth. For a moment, the huge skeleton said nothing, like he was processing the information. Then, for no reason, he burst in laughter, projecting some organic waste on Sans face. It was not a cute laughter. More of a morbid and very scary one. It creeped him out. Whoever this guy was, it seemed he lost it for a very long time.   
  
Sans sighed. What happened to Boss ? To Snowdin ? To him ? Or he really abused on mustard and he was living the worst bad trip of his life, or something bad happened to his timeline during the night. 

Whatever Papyrus laughed for, that didn’t answer his question. Even more creepy, he suddenly stop laughing like nothing happened and push Grillby’s door, to Sans’ surprise.

Seriously ? The Great and Terrible Papyrus entering the grease tank of the town willingly ? Damn, even if it was an other world, it was quite shocking. What happened to this Papyrus’ morals ? 

But Sans’ bewilderment didn’t stop here. He followed Papyrus inside, afraid of what he would find inside. It was even worse than the worst of his nightmares. All the bar tables had been gathering in the middle of the room and the biggest plate of spaghettis he ever seen of his life was placed on it. Most of Snowdin people were digging into the food like wild animals. No other term could define better what he had in front of his eyesockets. 

A burning smell hung in the air. Sans followed the black smoke before his eyes widened in pure horror. On the barbecue, a human child was grilling, their dead gaze fixed on him. He felt his inexistent stomach turning. He ran away from this horrible place and took a deep breath when he finally get outside.

— What the hell was this fucking sick shit ? he yelled at the emptiness. They all screwed up, god dammit ! Keep calm Sans, keep calm. There has to be an explanation. That’s not Boss. That can’t be Boss ! In what shit did I get myself into again ? Why the hell does things like this always happen to me ?

When he just started to believe nothing could be worse, a voice echoed at the entrance of the town.

— The Queen is coming ! Go hide, quick !

Grillby’s door blew up behind him. The entirety of Snowdin passed him over in utter panic. Villagers ran towards their house and shut shutters and locks behind them. Papyrus was the last to leave the bar. He caught his brother and threw him on his shoulder like a potato sack. They both sheltered in their house, then Papyrus locked the door and the windows. He seemed really scared, and Sans started to question whatever was happening now.

— Hey Boss, why are we hiding again ?  
— Because she’s coming ! The Queen ! Uncork your ear canals once in a while instead of slacking off outside !  
— The Queen ? What Queen ?  
— The Queen, yes ! Undyne !  
— What ? Undyne ? he repeated, bewildered. Boss, the hell is going on here ? Is there a role playing game or something that you didn’t warn me about ? Why everyone is freaking out because of some stupid alive sushi ?

Papyrus narrowed his eyes.

— Sans, are you sure you are fine ? You are really weird today.  
— Oh yeah, sure ! Obviously, I am the one who is weird here ! he suddenly yelled, exasperated. Pap’, I saw ya’ eaten a freaking human ! And Grillby is orange now ! And what the hell are ya’ stupid ugly damn shoulder pads ? What is this stupid costume ? Why everyone is scared of me ? And why the hell is everyone fearing Undyne ? All she does is watching childish cartoons with Alphys all day and faking to be mean to suck Asgore ! If this is some sick joke to make me go insane, well congratulations, that’s fucking working ! But that’s not funny anymore, ya’ asshole ! Turn back to ya’ shitty normal self now !

Papyrus seemed even more confused. Maybe Sans was right all along and that wasn’t his world. That would explain a lot of things. He fell on his knees and hide his head in his arms. He had to stay calm. Making a fuss won’t do anything more. He never thought he would so badly want Boss waking him up and yelling at him for being a moron. Damn, he could even hug him if it was happening now. 

Someone knocked twice at he door. Papyrus widened his eyes, terrified, and went to hide behind the couch. Sans could hear his bones rattling with fear. He sighed.

— Ya’ freak.

Tired of this nonsense, he decided to open the door. He came face to face with a brand new nightmare.

Undyne were standing here, frozen the second she saw him. Half of her face was gone. Someone or something cut her head like butter. She was also thinner, well, more like skeletal, and something bad were shining in her eyes. No doubt about it, this thing was nothing like his Undyne. The fish lady bowed the head and crossed his both eyes. She held out a trembling hand to the top of his skull, highly disturbed by something.

— How did you… How… she stammered in shock. We could… We have to…

Her face suddenly transformed to become more hateful and angry. She invoked a spear and rushed to pierce his skull. Sans saved his life only by teleporting a few inches to the right. He tried to slam the door, but it was too late. Undyne called the Royal Guard and put a bounty for whoever captured him first.


	5. Part 5 - Underfell

**Summary for the Chapter:**

> Papyrus is preparing his masterplan. He is sure of it : Sans will come home crawling and begging for mercy after the evil plan he just organised. Well... Or not.

#  **THE DOPPELGANGER**

###  _Part 5 : Underfell_

— Urgh, that’s so boring ! If you told me this morning I would wake up to cover the snow with mustard, I would have go back to bed ya’ know, Undyne whined again.

Papyrus lift his eyes up and let out a loud sigh. Squatting, his plastic gloves above his woolen gloves above his fighting gloves above another pair of plastic gloves was spraying the greasy substance that was burning his eyesockets in the middle of the forest. He was hoping that Sans, too addict to pass an opportunity like this, came home crawling and begging for Papyrus to spare his worthless life. 

A little further, the ropelgienger… dopplaranger… the thing that was similar to his brother was attached to a tree like a poodle. It was loudly masticating its sixth plate of frozen lasagnas of the day. Papyrus should have been flattered to see his cooking finally appreciated at its true value, however the thing were kinda eating everything that fell into its hands and it was worrying. Ten minutes ago, Papyrus even caught him capturing a rat and tearing off his head before eating it like it was the best meal it had for months. Just the thought of all the bacteria that rat’s head must contain gave him cold sweats.

At least, the skeleton refrained his murderous instincts towards Undyne. He was still growling like a bear when she came too close from him, but as long as she was just giving him more food, it seemed ok. That was the only thing this « Sans » seemed to know how to do anyways : eating. He refused to answer Papyrus questions in the laboratory and just looked at him blankly while itching that horrible hole in his head and asking for his axe back. For obvious safety measures, Papyrus didn’t give it back to him. He didn’t especially want a psychotic and armed monster roaming freely in Snowdin. There was way too much murderers out there. Even if « Sans » didn’t do anything yet, except trying to kill Undyne, which most of people down here tried one day or the other, Papyrus prefered to take no risk at all.

— I hope Sans is not hurt, mumbled Papyrus. Not that I care, he justified himself, but I don’t want to waste my precious medical supplies on his disgusting sorry face. I don’t like knowing him slacking off somewhere in the woods. It’s been two weeks he didn’t recalibrating my puzzles, and I’m going to make sure he’ll do it while crawling and crying like a baby the second he’s showing up.  
— That’s not like your puzzles were useful anyways, laughed Undyne. The human knows all the solutions for months now and everyone's tripping over them.  
— That’s not true ! Frisk loves all of my puzzles ! he yelled while crossing his arms, outraged. 

As he said the human’s name, the other Sans raised his head, troubled. And he got even more bewildered when a small kid in a striped sweater ran suddenly from nowhere, a big basket in his arms.

— Papy ! Papy ! they squawked, excited. Cake !

The kid jumped in front of Papyrus feet, the basket raising towards the head of the tall skeleton so he could take it.

— It’s for Sans and Papy ! the kid insisted, happily.  
— Oh, they’re calling you « Papy » now ? That’s cute, Undyne reacted behind them.  
— Shut up and keep spreading the mustard or I’ll going to put the entire bottle in your eyes, the skeleton growled.

Papyrus crouched next to the kid and opened the baskets. There was three pies in it : one looking like Sans, one looking like the kid, and one looking like him, even if his face seemed to have been flattened with a brick. Papyrus sighed dramatically.

— Human… I thought I got clear about flirting. Even if I am very big and majestuous, I can’t answered your high standards of kindness. But, alas, I, the great and terrible Papyrus, can’t resist the call of this sweet cookery. I feel obliged to accord you a second date after this. That’s the least I can do.  
— Please kill me for Asgore’s sake, Undyne whimpered.  
— Fr… Frisk ? suddenly asked a third voice.

The kid turned towards the other Sans and gasped in horror, pointing his head.

— Hurty Sans ? they asked with a trembling voice while pointing his head. Sans hurty ?  
— Frisk, is it really you ? the other Sans kept asking. Ah… Ah… But… That’s it then ? I’m finally loosing it ! Ah, ah. I’m crazy ! It took long enough but now I’m insane like the others !

He burst out laughing like a dement, creeping both Papyrus and Undyne out. Worry, Frisk was more brave. He came closer of the big skeleton and hugged him. Sans didn’t move, shocked for a second. Then he sniffed loudly before he started crying like a baby while hugging back the kid. 

Papyrus and Undyne watched the scene in shock. Both of them were out of words. Something was wrong with that monster. Very, very wrong.

— Wow, the fish lady said. You roppelgiengirer is a crybaby. I much prefer when he was trying to kill me. What a weakling !  
— Hum… Oh wait, my brother is also a sensitive weakling, Papyrus thought out loud. Maybe the human Frisk could help us find him ? It sounds like a new plan.  
— Urgh, why are we doing this again ? And who decided you were in charge anyway ? Am I not supposed to be your boss or something ?   
— Shut up, Undyne, you wimpey looser. Go find me a leash. We have work to do.


	6. Part 6 : Horrortale

**Summary for the Chapter:**

> Sans and his brother from an other world are fighting a very murderous and determined Undyne.

#  **THE DOPPELGANGER**

###  _Part 6 : Horrortale_

Sans never fought this hard before, and even less in his own house. Undyne entered by force with half of her Guards. They were trying to corner the small skeleton, but there was no way he was going without taking some of this bastards in the tomb. Papyrus had been tackle to the ground in the first seconds of the battle, which was worried his “brother” even more. He didn’t even fight ! What had that crazy sushi done to the other Sans anyway ? Well, he didn’t intend to stay here to find out. 

His eyes shined with a intense red and he fired a new salvo of bones in their direction. Two guards dusted in one touch as karma was doing his job. That was actually problematic. For such a result, these guys must have a very high level of violence, even more high than some of the best fighters monsters in his own world. Monsters couldn’t hold level of violence as good as human. In their world, except for the King that no fool would ever try to fight, monsters with the higher level of violence went insane and had to be killed to “protect” the others. He could feel his opponents crossed this line a long time ago.

Blinded with rage, the fish lady with her atrophied face charged him once again. The living room table paid the price of her madness and exploded as a wave of spears zapped it in pieces. Sans teleported behind her and went for her knees with a red sharp-cutting bone. He pierced it from part to part and she finally dropped on her side. Way was clear.

He rushed to the door before he stopped at the entry. He looked at Papyrus. Even if it was not really his brother, he felt bad leaving him behind. He drew in his last forces to make the two guards that was holding him back off. Two big red bones knocked them badly in the head and knocked them out. 

— We’re getting out of here ! Sans yelled. Hurry up you bonehead !

He rushed to the door. As he ran away, two strong arms caught him and he found suddenly himself on his “brother” shoulders. The door frame almost ripped his head off, but it didn’t matter as they were finally free. Sans thought for a bit, then turned Papyrus’ head towards Snowdin forest, where he found him sooner. If he wanted to find a way to get back home, that seemed a good beginning point.

Behind them, an angry roar resonated. Undyne, her valid part of her face melting and even more hideous than before, had tear off her leg for some reason. She was crawling toward them with determination, eye blood-injected. Sans and Papyrus exchanged an uncertain gaze before raise their shoulders and abandoned her to her fate. Anyway, as she insulted them, some heads appeared at the windows of the houses around. It seemed some people had one or two bones to pick with her. 

During fifteen minutes, the skeletons ran in the snow. Sans was guiding him to the place where he thought he woke up and was maybe enjoying a bit that, for once, Papyrus obeyed him blindly. If only Boss could do the same thing… Well… No. It was better Boss never knew where he went and what happened to him. He would straight kill him if he knew Sans had even thought to replace him for a few seconds. This idiot was persuaded he was unique and irreplaceable. His entire world woud fall into chaos if he discovered he was one version of Papyrus in the middle of hundreds of others.

Suddenly, a sly thought make its way to his head. What if the Sans of this universe took his place in his own world ? The world he was in right now was even more shitty than his original one. There was no way he stayed here forever. Even if he could understand that the other Sans could think the same thing, he won’t let him stole his Boss, as lousy, cold, paranoid and angry he was. He could even kill his clone. He didn’t like doppelgangers and wouldn’t hesitate to make him understand. 

Well, of course, for that, he would have to get home first. He hoped it was still possible. 


	7. Part 7 : Underfell

**Summary for the Chapter:**

> With Frisk's help, Papyrus hopes he can find Frisk. But someone is not very happy with how the tall skeleton is treating the child.

#  **THE DOPPELGANGER**

###  _Part 7 : Underfell_

Papyrus was standing next to the spot where he first found the other Sans. He was waiting impatiently. He had passed a collar and a leash around Frisk neck, who didn’t seem to be bother about it, and the skeleton was pointing the ground with insistence, hoping that the kid would sniff it. Behind the child, a huge form of the other Sans watched almost like a protective guard dog, which was making both Papyrus and Undyne very uncomfortable. The fish lady was less and less enthusiastic about Papyrus plan, but she was bored anyway and decided to follow them.

— Come on, Frisk ! Find Sans ! said Papyrus with an extremely exaggerated cheerful voice, while throwing one of Sans’ coat at them.

The kid fell on the buttocks, carried away by the clothing’s weight. Papyrus sighed and impatiently pull on the leach to make Frisk do something useful. The kid whimpered when he fell on his knees under the harshness of the skeleton. 

That was too much for Sans. Suddenly, the skeleton caught the leach and bite in it with so much force that it broke in two. Boss, not used to disobedience and rebellion, turned towards « Sans » a dark, angry and murderous gaze. 

— What ?! growled Sans at him. You hurt the kid. Do not look at me like that, stupid looking pile of bones.   
— Excuse me ?! roared the other skeleton.  
— Yeah, excuse you. Touch the kid again and I’ll show you how I greet people on your kind in my own home, Sans said in a threatening tone.

Undyne slightly hid behind Papyrus, very uncomfortable. Papyrus opened and closed his mouth several times, searching for something to answer back. That was new. Sans, his Sans, would never opposed publicly like this to him. Papyrus was not sure to know how to react to this. 

In front of them, his “brother” quietly picked up the kid… and left towards the woods without according them more attention. Frisk didn’t struggle, scared by the size of his carrier. 

— Eh, Pap’, where is he going ? asked Undyne. If he killed the kid or something, Toriel gonna have our heads !

Papyrus did not answered and followed him, a bit alarmed. He invoked a bone and ran after him. Sans ignored him completely, concentrate on a rock cavity. He scratched his cheek before he touched the rock, testing his solidity, then kicked it with his feet. His leg, then his body, disappeared in the stone, carrying the kid away with him. 

Papyrus stopped for a bit, shocked, but quickly recovered. He had to save Frisk, no matter where he was going. He touched the rock and was absorbed in it. 

When Undyne finally arrived, dragging her feet, the three of them were gone without a trace. The royal guard sighed loudly, then yelled at the emptiness.

— I swear to Asgore that if you’re dead Papyrus, I’m stealing your fucking house !

But nobody came.


	8. Part 8 : Horrortale

**Summary for the Chapter:**

> Sans is still tyring to find a way out of this damn universe. But maybe the solution all this time was to wait patiently for the rescue to come.

#  **THE DOPPELGANGER**

###  _Part 8 : Horrortale_

Sans was wandering in the forest for almost an hour now, this ugly Papyrus clone and his big sharp teeth close to him. Even if he had a precise idea of where he appeared in this universe in the first place, nothing helped him to find a way back home in this place. There was only snow, trees, and snow again. Everywhere and out of sight.

He was not really helped anyway. Papyrus, who still seemed to think he was his brother despite still having two eyes, which the other Sans apparently didn’t, complained from times to times about the meal he was forced to abandon at Grillby’s and how he wanted to get back and finish it. His appetite for humanity was creeping Sans out. What could have possibly happened to monsters so they could have give in all their mercy and kindness ? He understood by now that it had something to do with Undyne, but she couldn’t have fucked up an entire world just by herself, right ? 

He was well place for knowing that no one would voluntarily eat Papyrus’ pastas without being forced too. Everything felt so bad here he wished his world never became like that despite how bad he was already. Better murder Undyne quickly than seeing that happening. Even if he didn’t imagined his Undyne, the same who was rhapsodizing over kitten pictures on Frisk’s phone while cutting bad guys head, ever turned as bas as the one from this universe, he couldn’t get rid of that obsessive thought. It was worrying.

Discouraged by the failure of his initial plan, Sans soon give up and just let himself fall in the snow, arms and legs ecarted. 

— Sans ! grumbled Papyrus. What are you doing ? This is no time for a nap !  
— Listen, man. Ya’re not my Boss and I’m not yar Sans. I have no order to get from ya’. I sleep if I want too. Piss off. 

Papyrus stomped angrily. Then he took his hand and pull on it to force him to stand up. Sans giggled while trying to complicate his job and letting himself fall on his back. Well, at least until a human-looking form in a red and black shirt crossed the forest, crying and yelling for help while looking nervously behind him. Sans gasped and jumped on his feet. He pushed Papyrus on the ground and ran towards the kid.

— Frisk ! Frisk, I’m here !

The kid stopped at a good distance from him, suspicious. When their eyes finally met Sans eyesocquets, they sighed in relieved and ran towards him. The child was almost reaching him when two big legs passed behind Sans’ head and jump on the tiny hume that shrieked in terror. Papyrus tightened the grip around Frisk body and rolled in the snow with it like how a wolf captured a deer.

— I got it ! he gloated. Sans ! Look ! I captured the human ! Sans ! … Sans ?

When Papyrus lift his head, two huge blasters were circling the tiny skeleton. All light had disappear from Sans eyesockets. He was done fooling around. Terrified, Frisk was yelling and struggling in the big skeleton arms, calling Sans for help again and again.

— Put the child down, warned Sans with a deeper aggressive voice.

Papyrus blankly watched him, confused, then he let go of the child. Frisk fell in the snow, before running towards Sans and hide behind him. The skeleton could feel his hand shaking, grabbing his coat. 

— Ya’ okay ? asked Sans. He didn’t hurt you ?  
— Am… Am fine, Frisk answered.  
— Pap’ ! yelled a third voice deeper in the forest. 

Papyrus twisted around. Sans gasped in surprise. The newcomer was a second skeleton that was looking exactly like him, at the exception of a big ugly hole in the right top of his skull. He looked bigger too. He was holding a huge axe in his two arms and seemed very nervous. He stopped, then his only eye turned towards the blasters, then towards his brother before his red cat-eye pupil narrowed suspiciously, fixed on the other Sans and the kid.

— Who’s that ? he asked aggressively to Papyrus.  
— Well, I thought that was you, but it seemed I was wrong. So I don’t know. Oh ! But Sans, he has a human ! And I captured him ! But then he yelled at me so I let it go because he was scary.  
— Oh really ? What an asshole. Hey you ! he said to the other Sans with a threatening voice. That’s my human. My brother captured him first. Go find an other one.  
— Listen ya’ overgrown looking shit, I don’t know what the fuck is happening in yar universe, but that’s my kid. And if ya’re even think about eating him or something, I’m gonna give ya’ hell of a good time. And his mom too, she is pissing off very quick when ya’re mean to Frisk. Push me and I show you how I handle shitty people like you in my world.

Frisk suddenly pulled Sans jacket and pointed something behind the two others skeletons.

— Papy ! Papy ! they said, excited.

Indeed, Boss was standing there, behind that simulacre of a Sans, in all his highness and majesty, and with his stupid looking pointy shoulder pads. And damn, he was not happy at all.


	9. Part 9 : Horrortale

**Summary for the Chapter:**

> Boss is here. And he is not happy.

#  **THE DOPPELGANGER**

###  _**Part 9 : Horrortale** _

Papyrus looked at the roppel… doppler… thing-ganger, then to the other Sans. Even if there was no way he said that out loud, he was relieved to have the fragile and non interesting silhouette of his brother back in sight. Frisk was hiding behind him and two enormous blasters was protecting him from the other pair of brothers. Indeed, between them stood an other Papyrus, as broken as the other Sans. His decayed teeth pointed towards several directions and he looked like a perfect moron. Of course he was, what would he even wait from that ? There was only one perfect Papyrus and it was him.

— Hey B-Boss… stammered Sans. I didn’t thought ya’ would come and get me.  
— I’m not here for you, I’m here for Frisk, corrected Boss.

The blasters disappeared and Sans bowed his head in what Papyrus assumed was shame. Good. As always, Sans was crushing himself in front of his magnificence. Papyrus swelled the chest and stepped forward with insurance. He stopped between the ugly clones and his stupid brother, as the valiant savior he was.

— I don’t know who you are or what you want, he said, but I highly recommend you to get out of here before I, the great and terrible Papyrus, force you too. The situation might not be so advantageous for you in a few seconds. I have a torture room and I know how to use it perfectly.  
— Hey Sans, interrupted his clone. Why does he have high heels ?  
— Because he’s a wimp, the interested mocked. Seriously, bro, look at his pad shoulders. He smells the oversized ego and the spoiled baby. Not even a piece of meat on his bones to be useful for us, he regretted.  
— Sans, we talked about this. That’s mean, brother. Maybe his brother never taught him how to wear good shoes, that’s all. Anyways, are we catching the human or what ?  
— Of course, brother. I let you attack first. You’re the coolest, remember ?

A salvo of bones rushed towards the other skeletons. Papyrus raised a bone barrier in front of him and took the attack. 

— Sans ! There is a rock not far away over there, he said, pointing an approximative direction. Go through and go home. I highly recommend you not to mess up and keep the human Frisk alive. You don’t want Miss Toriel to dust you back home, do you ?

Sans swallowed. Nope, no one would want that. He caught the kid by the hand and dragged him behind towards the place his brother indicated. 

The ugly Papyrus broke Sans’ brother bone barrier and attacked him at full force with a bone sword. The red Papyrus parried the attack and throw the other one back to the ground. One blaster shoot at him but he avoided it by a jump on the side. By this, his arm blocked the other Papyrus neck and Papyrus could see in his brother’s eyes that it was really not pleasing him. 

— Let my brother go, he growled menacingly.  
— Sans ! yelled his “prey”. The human is running away !

The skeleton kicked his opponent in the knee, forcing him to release him. The huge skeleton rushed toward the other Sans, running away with the kid. Papyrus was now alone with his brother’s copy, who visibly was in a bad mood.

— He’s going to catch the kid, and you, you’re gonna get back home.  
— Frisk has already a family.  
— Frisk doesn’t need a moron on high heels who’s treating him like a damn dog. If you want him back, you’ll have to go through me first.

Papyrus smiled sadistically.

— Oh, but that will be my pleasure.


	10. Part 10 : Horrortale

**Summary for the Chapter:**

> Sans and Frisk are trying to run away from this universe. But Papyrus' doppelganger still wants that human.

#  **THE DOPPELGANGER**

###  ** _Part 10 : Horrortale_ **

— Sans ! Sans ! Rock !

Holding like a koala on the skeleton back, Frisk was guiding him approximately trough the woods. They chose the wrong path twice already and good old Sansy was slowly loosing his temper, fortly annoyed by Frisk kicks in his ribs to make him turn right or left like he was some sort of pony. 

But this time, the kid seemed confident and pointed a big pile of rocks a little further. Sans rushed to get in there. His legs started to hurt. This day had been grueling in every way and he now obsessed with one unique thought : getting home, collapsing in his bed with a mustard bottle and sleeping for at least two days. Boss could still trying to move him, there was no way he was doing anything after that.

— Sans ! Bad Papy ! Look !

The all accompanied with a hit on his right side. Sans growled and turned his head just in time to see his brother clone running towards them, drool on his jaws. 

— Shit. Are you sure that was these rocks ? he asked to them in a hurry.  
— Yes ! Rock, rock !

Sans nodded. He gained momentum and ran at full speed towards the stone. In an ideal world, he would have go through and get back home. Instead, he majestically ate the rock wall painfully and fell back on his pelvis, half stunned. He gazed menacingly at Frisk who make himself very small. Shyly, he pointed next an other rock, a few inches on the right, which was indeed shining strangely. 

Unfortunately, they couldn’t make it. The other Papyrus jumped suddenly on them, shouting scarily, then he tackled Sans to the ground and put a sharp bones on his neck. The skeleton tried to struggle, but like his own brother, his assailant had a titanic force. Sans could barely breathe, Papyrus’ hand was crushing his chest. 

— Let… Let go, he moaned painfully. I… I surrend…

A pine cone slammed curtly against Papyrus’ face. As surprised as Sans, the huge skeleton turned slowly his hand toward the human. Frisk, determined, had already pick up several new munitions. 

— Bad Papy ! Bad !  
— What ? How dare you ? he replied, outraged. Human ! He was the one who kidnapped you ! he said, pointing Sans under him. I am only saving you ! My brother doesn’t like when strangers stole his humans.  
— Ya’ damn psychopath, growled Sans, while he tried to free one of his arms. F… Frisk ! Go through the rock ! Now !

Frisk dropped everything he was carrying and decided to obey. But Papyrus didn’t agree. Before he could reach the portal, he let Sans go and jumped on the kid. Frisk screamed and struggled in his arms. Desperate, they even tried to bite his arm. This wasn’t very effective but diverted the skeleton enough so Sans could get up. 

Angrily, the tiny skeleton threw himself on the huge one legs and started to hit them with a bone to break them. Papyrus sighed. He cleared his arm off and hit him with the bone of a club size. Sans flew back and slammed against a tree, a huge crack on his skull.

— Sans ! yelled Frisk, horrified. 

They kicked Papyrus ribs with their feet and forced him to drop them in surprise. The kid ran next to their friend and shook him to wake him up so they can both finally get out of here. 


	11. Part 11 : Horrortale

**Summary for the Chapter:**

> Horrortale Sans and Underfell Papyrus are fighting in the forest while their brothers are away. But something fishy interrupts them.

**Notes for the Chapter:**

> We're almost at the end :D The last two parts are coming tomorrow ! Thanks for the support !

#  **THE DOPPELGANGER**

###  ** _Part 11 : Horrortale_ **

Sans’ clone was clearly more energetic than his own brother. The fight was engaged for only several minutes and the little skeleton already succeded twice to break Papyrus defenses. The skeleton could try as hard as he could and use all of the skills he learned during his long years as captain of the Royal Guard, he was still far away to have learn everything on the job. Undyne was kinda right : this Sans was better at fighting than his own brother. He was more bestial, more wicked, more cunning… Well, more everything to say the truth. If this one had been born in his world, he would have become a terror in no time. 

After a new charge, both fighters, out of breath, decided for now to stay with a verbal fight.

— Not bad for a Papyrus, Sans said. Couldn’t teach a thing to my bro. Well… We obviously didn’t have the same Undyne obviously.  
— Home, it’s kill or be killed. I didn’t exactly have the choice. Sans taught me how to fight, then I engaged myself in the Royal Guard so I could take the place of the wimpy Undyne. I can easily beat her down now, he boasted proudly.  
— Eh, eh, Sans laughed. Here, you’ll be dead before you could even get close to her. Survival before everything else. Eat… Or be eaten. You should be ready to the idea. My world was like yours before. And then, one day, the kid killed Asgore and vanished, never to be seen again. Monsters started eating each other… Then humans. That’s the law of nature, the food chain. The strongest lives. The others… Well… They better hide.

An enraged growling could be heard next to them. Papyrus recoiled. Undyne, or more exactly what stayed of her was crawling towards them, with a crazy look and the face inflated by hematomas. Well…What was left of her face. The skeleton looked at Sans clone. He seemed suddenly very nervous and agitated, almost… scared ?

— Is it because of her you were scared of my Undyne ? he said with a more calm voice.  
— Eh, I wouldn’t trust her if I were you, he said in a low voice. One day she is training Pap’, playing with the kid, and the next day she is taking control over the Underground and forcing everyone to become soldiers. The CORE stopped working here, and she decided to find an other power source. Alphys told her about our magic eyes. She didn’t hurt Papyrus because she is a coward, so she embushed me and…

He pointed the huge hole on his head. Papyrus couldn’t refrained a shiver. The fish lady pushed on her only leg to get up and burst with a deranged laughter.

— Oh yeah ! That’s my fault ! she laughed hysterically. If you let us worked, we would have succeed ! But no… No ! That asshole lobotomised Alphys and now we’re all stuck here because he was too selfish ! But that’s not a problem. I saw the human. With its determination, we could finally start everything again !

She pointed towards the forest. The other Papyrus was coming back, Sans in one hand, badly injured to the head, and Frisk in the other one, growling and struggling to escape in his arms. Papyrus sighed. Of course Sans would fail. Well, nice try. Papyrus took a step forward to face Undyne.

— Well listen, you ugly fish. Even if we’re not from the same universe, I would have done the same thing than this Sans. And my idiot of a brother too. I won’t pretend I know what you all went through, but I prefered to let Frisk to these dopp… dapp…   
— Doppelgangers, whispered his brother’s clone.  
— Yes, I prefer to let Frisk to these things than in your hands.  
— Um… Sans ? asked his own clone. Are they our friends now ? I’m confused. Because, you know, I just knocked out this one for no reason, he said while lifting the Sans he was carrying by his jacket. That’s not a really good first impression.  
— Welp, answered the Sans with the hole in his skull, Spoopy-Angry Face is right, we can’t let her catch the kid, right ? Pap’, we’re gonna kick her ass. 

Undyne bursted in laughter.

— So that’s it ? You’re thinking than just because you found a clown in high heels I’m gonna kneel in front of you ? You don’t know who I am ! I am Undyne, queen of the Underground and I’m gonna…

A pine cone crashed in the middle of her forehead. She instantly went silent and shot Frisk with her eyes. Back on the ground, the legs trembling, the kid was hiding behind their Papyrus, nevertheless with a determined gaze. 

— Papy stronger ! they said on a challenging tone.

Papyrus smiled, delighted, then charged the ugly fish without any regret.


	12. Part 12 : Horrortale

**Summary for the Chapter:**

> A fight with Undyne later...

#  **THE DOPPELGANGER**

###    
**_Part 12 : Horrortale_**

Locked up in a red and blue bone cage, Undyne was enraging, muzzled by the red scarf of her universe’s Papyrus. The two pairs of brothers were walking calmly towards the magic rock, because two of them were not really at their place in this horrifying and scary world. Frisk, more relaxed, was happily holding hands of both Sans, smiling. In the end, everything had been resolved in an almost-pacific way. Well… If throwing colorful bones at a one-leg fish was considered as pacifist.

— What’s going to happen to her now ? asked the Sans in his black jacket. Ya’ gonna release her ?  
— We’ll see, answered his clone. We’re going to search our true Queen in the Ruins, and she will have a fair trial. After that, well… he sighed. I guess we’ll get back at eating dandelions and garbage until our Frisk finally decides to show up.

Frisk hugged him. The kid was sad for him.

— Sans must determination, they said cheerfully. Frisk come back. Sure.

The big Sans smiled and rubbed their hair.

— If it was that simple… I’m… I’m sorry I tried to kidnap you. That wasn’t very nice of me. It seemed someone really cares about you, despite how shitty they look.   
— Frisk forgive.  
— Yeah… Ya’re forgiving too many people, growled their Sans. I’m reminding ya’ that at home it’s kill or be killed. We’re gonna work on that.

A bonely hand taped his shoulder.

— My dear brother, react the Papyrus with shoulder pads, we are twenty-eight minutes late to give Frisk back to his mother. If you don’t want the both of us being executed when we get home, could you please hurry the fuck up ?!

Sans sighed loudly with annoyance.

— Ya’ heard the Boss, he said to his doppelganger. If that magic rock is still working after that, feel free to pass by and stole lasagnas from the fridge at home. There’s too many anyway. And it could add some animation in this stupid Underground.  
— The same if Spoopy-Angry Face is becoming too much, answered the other Sans, pointing at Papyrus. You should go now. My Papyrus is hungry anyway, he can be tetchy without his damn spaghettis.  
— Was this really a human roasting at Grillby’s ?  
— Yeah… That’s complicated.  
— SAAAAANS ! shouted a voice in the forest. The spaghettis will get cold !  
— I’m coming, bro.

Sans smiled a last time at his clone then peacefully turned around and disappeared behind a big tree. The other Sans shrugged and entered the portal.


	13. Part 13 : Underfell

**Summary for the Chapter:**

> Sans and Papyrus are finally getting home. But someone is waiting for them and is not very happy.

**Notes for the Chapter:**

> Here goes the last two parts of the story ! Thank you for your support on this fanfiction, it was really nice :D A way bigger project is coming very soon :3 See you soon and enjoy the end of that little crossover :D

#  **THE DOPPELGANGER**

###  ** _Part 13 : Underfell_ **

— Mom !

Frisk ran towards her mother open arms at the exit of the magic rock. Sans and Papyrus widened eyes of surprise. They didn’t planned to meet her so soon. Undyne was behind her, visibly very nervous.

— Could I know the hell you were ? Toriel angrily yelled at them. Undyne told me there was a doppelganger of Sans somewhere here. Where is he ? Did he hurt my child ?  
— N-No, shrieked Papyrus with an abnormal high speech voice. It… It was just Sans. He got hit badly on the skull. You know how stupid he can be.  
— What ? roared Undyne. You got back your brother instead of him ?! But I was going to ask him if he wanted to join the Royal Guard ! … Hey, Pap’, can I at least break his skull so he can look less ugly ?   
— Do that and I ate yar fingers like appetizer cookies, threatened Sans, his eyes narrowing.

Papyrus stepped between them and cleared his throat.

— We’re sorry for the delay, Miss Toriel. That won’t happen again.  
— Papy nice, Frisk tried to calm the murderous instincts of his mom. Papy saved Frisk from bad Papy.  
— Fine, you can go for this one and only time, she said with a dark voice. But if I find one bruise on them, I blow both your heads off.

Sans and Papyrus swallowed nervously. She turned around without one more word. Frisk waved them with his hand and a sorry face. The skeletons and the fish lady waited them to be out of sight before taking back the way to Snowdin.

— Uh… Hey, Papyrus, said Undyne with a high speech voice. Ya took yar time to come back so… Are you going to kill me if I tell you I sold your TV to a random Temmie who was passing in town ?

Papyrus stopped, sighed, then turned his head towards her.

— You’ve got exactly five minutes and thirty-three second to get it back before I come to break your knees.  
— Understood, Boss.

She ran away in a hurry through the forest. Sans laughed next to him, before he went silent when he crossed his brother dark gaze.

— As for you, Sans, you are deprived of mustard for a month. Get stuck in this other universe again and I swear I let Undyne crush your skull. Did I make myself clear ?  
— Y-Yes, Boss. I promise I’ll behave.

Papyrus smiled, then walked towards Snowdin without one more look at him. Sans smiled discreetly. Even if he would never confess, Papyrus had done all this way just to get him back. Maybe he was still caring about him after all ? Eh, who knows. Maybe Sans would ever have a good evening for once. 

**THE END.**


End file.
